Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

PO is now at the Physics Department of Bielefeld University!

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

205 submissions , 163 unreviewed
5,047 questions , 2,200 unanswered
5,345 answers , 22,709 comments
1,470 users with positive rep
816 active unimported users
More ...

  The curvature of the space of commuting hermitian matrices

+ 6 like - 0 dislike
1040 views

This is a question that I asked in the mathematics section, but I believe it may get more attention here. I am working on a project dedicated to the quantisation of commuting matrix models. In the appropriate formalism this problem is reduced to a quantisation in a curved space -- the space of commuting matrices. The general prescription for quantisation in curved space involves ambiguity of the Hamiltonian operator proportional to the scalar curvature of the curved space - hence my question.

A set of $p$ commuting $n\times n$ hermitian matrices $X^{\mu}$ for $\mu=1,\dots p$, is parametrised in terms of a set of $p$ diagonal matrices $\Lambda^{\mu}$ and an unitary matrix $U$ via:

$X^{\mu}=U\,\Lambda^{\mu}\,U^{\dagger}~~$ for $~\mu=1\dots p$,

clearly not all degrees of U contribute to this parametrisation, for example a reparametrisation $U'= D\,U$, where $D$ is a diagonal unitary matrix would result in the same set of commuting matrices. In other words only the elements of the quotient space $U(n)\,/\,U(1)^n$, which is the maximal flag manifold $F_n$, contribute to the parametrisation. The metric on the resulting curved manifold can be calculated as a pull-back of the metric on the space of hermitian matrices defined as:

$ds_{X}^2=Tr\,\left( dX^{\mu}dX^{\mu}\right) $ ,

Using that $U^{\dagger}d X^{\mu} U=d\Lambda^{\mu}+[\theta,\Lambda^{\mu}]~~$, where $\theta$ is the Maurer-Cartan form $\theta=U^{\dagger}dU$, one can write the induced metric as:

$ds^2=\sum\limits_{i=1}^n(d\vec\lambda_i)^2+2\sum\limits_{i<j}(\vec\lambda_i-\vec\lambda_j)^2\theta_{ij}\bar{\theta}_{ij}~~$, where $~~\vec \lambda_i =(\Lambda^1_{ii},\dots,\Lambda^p_{ii})$ .

Now I need the Riemann curvature of the above metric. It seems that it is convenient to work in tetrad formalism, using tetrads $E_{ij}=|\vec\lambda_i-\vec\lambda_j|\,\theta_{ij}$, for $i<j$. The problem is that $d E_{ij}$ will now contain a term proportional to $(\theta_{ii}-\theta_{jj})\wedge\theta_{ij}$ and since $\theta_{ii}$ are not part of the basis the spin curvature cannot be written easily without using the explicit parametrisation of $U(n)$. Intuitively, I know that the scalar curvature should depend only on the lambdas ($\vec\lambda_i$), and I have verified that explicitly for $SU(2)$ and $SU(3)$, however a general result seems to require some invariant way to express the pullback of the term $(\theta_{ii}-\theta_{jj})\wedge\theta_{ij}$ on the submanifold spanned by the off diagonal $\theta$'s.

I was wondering if mathematicians have explored the manifold of commuting hermitian matrices. In fact even a reference to a convenient parametrisation of the maximal flag manifold $F_n$ would greatly help me in deriving a general expression for the scalar curvature. Any comments/suggestions are welcomed.

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user vesofilev
asked Apr 28, 2014 in Theoretical Physics by vesofilev (30 points) [ no revision ]
Crossposted from math.stackexchange.com/q/773476/11127

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user Qmechanic
The space of $p$ commuting $n\times n$ matrices is not connected, since a unitary transformation does not change the rank of a matrix. Each connected component corresponds to a specific choice of the ranks of the commuting matrices. The connected components are generalized flag manifolds. Since they are homogeneous spaces, they are of constant curvature and there are many ways to calculate their curvatures.The following lecture notes may be of help to you uregina.ca/~mareal/flag-coh.pdf

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user David Bar Moshe
Thanks for the comment. I have read parts of this paper before. I am a bit confused by your statement though. One can easily work out explicitly the 2×2 case: Upon redefining $\vec\lambda_1−\vec\lambda_2=\sqrt{2}\vec r$ and $\vec\lambda_1+\vec\lambda_2=\sqrt{2}\vec y$. And further change to polar coordinates in the space spanned by r⃗ one obtains:

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user vesofilev
$ds^2=d{\vec y}^2+du^2+u^2\left(d\Omega_{p-1}^2+d\Omega_2^2\right)$, where the vector arrow represents induces running from 1 to $p$ for the number of commuting matrices and $u=\vec r^2$. Now I can see that this space is not simply connected, but it is connected and it does not have a constant curvature, rather it is proportional to $1/u^2$.

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user vesofilev
@vesofilev: The $SU(2)$ case is a special case. Take for example the space of one commuting matrix in $SU(3)$, the spaces obtained from the adjoint action on $diag(0,0,1)$ and $diag(0,1,1)$ are disconnected.

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user David Bar Moshe
For one commuting matrix, it is just a flat space, because it is just a change of co-ordinates. The thing is you have to vary the elements of the diagonal matrix to. If we follow the logic from your last example, even $R^3$ would be disconnected. You can take two planes extended along $x$ and $y$, but separated in $z$. These planes are indeed disconnected, but the whole space isn't. I think your example is similar.

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user vesofilev
In other words the matrices $diag(0,0,1)$ and $diag(0,1,1)$ considered as points in the space of diagonal hermitian matrices (which is the same as the space of real diagonal matrices) can be connected, because the space of diagonal real matrices (which is $R^3$) is connected

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user vesofilev
@vesofilev: Sorry for not understanding you. The following reference arxiv.org/abs/0801.2913 contains a parameterization of the matrix $U$ for the flag manifold $\frac{SU(3)}{U(1)\times U(1)}$ on page 9, the third paragraph. (The authors call it the dressing matrix).

This post imported from StackExchange Physics at 2014-05-04 11:28 (UCT), posted by SE-user David Bar Moshe

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
p$\hbar$ysi$\varnothing$sOverflow
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
Please complete the anti-spam verification




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...